I have a list with a Date Column and a Followup column. If the weekday of the date column is Thursday, the follow up should be on Monday next week. If the date column is Friday, follow up on Tuesday, if Saturday or Sunday, follow up on Wednesday.

If Date column is Mon,Tues,Wedn, follow up + 2 days.

So far I have created a new column to retrieve the weekday by name. =TEXT(WEEKDAY(Date);"dddd")
But I just don't know if the more complex calculations can be done within the column or if I need a workflow.

Nice, it is also good for them to know the number system behind WEEKDAY (numbers 1-7 = Sun-Sat) 1-Sun, 2-Mon, 3-Tue, 4-Wed, 5-Thu, 6-Fri, 7-Sat. And when they replace your "Datum" to be sure to put square brackets around it like this [Datum]
– Chris GFeb 9 '18 at 17:46